The present invention relates broadly to a position transducer apparatus, and in particular to a digital transducer apparatus.
Aircraft and missile flight control systems utilize heavy duty high speed servo actuators to perform control functions. The electronics for these servo control loops have long been strictly analog. However, the outer loop flight control calculations have been performed with digital computers in recent times. Servo loop closure has continued to be analog for various reasons, some of which are that analog computation by dedicated elements at very high speed and prior guidance computers were too slow. It was a common belief that servo commands had to be analog. Most systems utilized servo feedback mechanisms having an analog output and digital shaft encoders were not competitive in a systems sense. Furthermore, digital electronics had been too costly and digital actuators too unwieldly in most system applications.
In the prior art, position transducers were utilized that were cylindrical in shape with an internal movable slug. They were mounted in the center of an actuator body and shaft, the transducer body was fastened to the actuator body while the transducer slug was fastened to actuator shaft. This packaging was very nearly ideal.
The present transducers are wired as a transformer with a primary excitation winding and two secondary windings which buck each other and are connected in series. The internal slug equally flux couples the opposing secondary windings at center or null. One or the other secondary winding dominates depending upon the direction and amount the internal slug is moved. The resultant output changes phase as the slug crosses center and increases amplitude proportionally as the slug extends. The transducer output is demodulated to a DC voltage proportional to the stroke which is used directly with analog electronics. If used with digital electronics the proportional (analog) DC voltage must be A to D converted prior to use.
Digital encoder methods utilizing shaft encoders have been tried in various applications. However, in most all flight control servo applications, the shaft encoders must provide absolute value as compared to incremental value and therefore require a Grey code readout with Grey code semiconductor decoding. A further problem is the difficulty of packaging and the many wire outputs. The wire problem can be solved, at a price if a nearby data bus electronics is available. However, in a hardwired system the number of wires create severe limitations. In addition, the making of a linear encoder is more difficult than a rotary one and packaging it inside an actuator with an opaque fluid at 3000 psi further compounds the difficulty. The present invention provides a viable alternative.